Ecology, nutrient reserve
dynamics and movements of white-
faced ducks in South Africa
dynamics and movements of white-
faced ducks in South Africa
SA Petrie & KH Rogers. 1997 Department of Environmental Affairs and Tourism. Pretoria ISBN 0-621-27962-5
Executive Summary
Background and Motivation Widespread semi-arid conditions, wetland drainage and increased water abstraction have reduced the availability of natural wetlands to waterfowl in areas of South Africa. The Nyl River floodplain provides an archetypical example of the problems facing most of South Africa's ephemeral wetlands. It has continually been threatened by damming and development which has the potential to reduce the periodicity, duration or intensity of flooding events, and consequently, its value to breeding waterfowl. The White-faced Duck (Dendrocygna viduata) is endemic to and widespread in Afrotropical, Neotropical, and some semi-arid regions of sub-Saharan Africa, and is the most abundant Anatid species on the Nyl River floodplain. Despite its wide range, the White-faced Duck is analogous to other South African waterfowl species in that its breeding and wintering biology as well as its movement patterns have not been adequately studied to date. Therefore, the expansion and improvement of pure and applied waterfowl research effort in South Africa must act as a prelude to the effective management and conservation of South Africa's Anatids and their habitats. While the Nyl River floodplain is an important breeding area for White-faced Ducks, events on breeding grounds only constitute half of the annual cycle of waterfowl. Based on studies of north-temperate species, it is being increasingly recognized that events while on wintering areas have an influence on future reproductive output and survival in waterfowl. Therefore, it is manifest that to fully understand annual cycle events, and how and where nutrients are acquired to meet associated costs, waterfowl must be studied throughout the entire annual cycle. This will provide information necessary to effectively manage and conserve South Africa's waterfowl and the wetlands that they utilize. In view of the aforementioned paucity of scientific information pertaining to South Africa's waterfowl and the wetlands which they require, as well as the immediate and ever present threat of damming, drainage and development of many of South Africa's wetlands, the need was expressed by researchers from the Centre for Water in the Environment, University of the Witwatersrand, to study the biology of the White-faced Duck throughout its annual cycle. The breeding season biology (diet, activity budgets, moult, nutrient reserves) of the White-faced Duck was studied on the Nyl River floodplain. Given the lack of White-faced Duck movements information, movement patterns and wintering sites were studied and identified as a prelude to studying wintering season biology. Given recent advances in satellite transmitter technology, use of platform terminal transmitters (PTT's) was deemed the best option for determining the movement patterns and identifying the wintering areas of White-faced Ducks. Experiments were conducted on captive birds to determine the most suitable transmitter attachment technique and identify any adverse effects that harness-attached transmitters have on activity patterns or condition. Confident that teflon harness-attached satellite transmitters were suitable for tracking White-faced Ducks, two post breeding birds were tracked from the Nyl River floodplain using satellite PTT'S. The wintering season biology (diet, activity budgets, moult, nutrient reserves) of White-faced Ducks was subsequently studied on agricultural areas within 100 km of the Nyl River floodplain where satellite tracked birds spent the winter, as well as on more "traditional" wintering areas in northern KwaZulu Natal. The science of waterfowl biology has been excessively concerned with those duck species that occur in north-temperate regions where the availability of waterfowl habitats and foods are highly predictable throughout the annual cycle. This has resulted in the development of several biological paradigms of duck nutrition and the timing of annual cycle events which owe their derivation solely to species of north-temperate regions. This is somewhat problematic, as semi-arid systems are much less predictable than north-temperate, which probably has a distinct influence on the spatio-temporal availability of waterfowl habitats and foods. Therefore, another key motivation for the study was that it provided the unique opportunity to compare and contrast the life-history strategies of a species from southern hemisphere, semi-arid, with those utilizing north temperate systems. While investigating the life history strategies and ecological requirements of the White-faced Duck, the study also identifies future research needs and possible management alternatives for the species. The study comprised three main sections: breeding season biology, movement patterns, and wintering season biology, each of which constitutes one of three major sections of the report. Key Objectives and Conclusions White-faced Duck Breeding Biology- Adult and juvenile diets in relation to sex, age and reproductive stage. Adult and juvenile White-faced Ducks were collected to determine which dietary items are important during reproduction and duckling development and to ascertain what portion of the diet consisted of animal matter. Males and females consumed similar proportions of plant and animal material throughout all stages of reproduction. Breeding White-faced Ducks were primarily herbivorous, as plant matter constituted over 99% of the diet, the proportions of which remained constant during pre-rapid follicular growth, pre-ovulation, laying, and incubation. Invertebrates comprised 6.6% of Class IIB duckling diets. Agricultural grains were the most common dietary items of Pre-RFG birds (91.9%) on an aggregate percent dry mass basis. The seeds of Panicum schinzii, a native terrestrial gramminoid, dominated the diet of adults during RFG (51.7% dry mass), laying (65.2%), and incubation (91.8%). The fact that White-faced Ducks were physiologically capable of reproducing whilst consuming small quantities of animal matter enabled them to breed on wetlands which support sparse macroinvertebrate populations relative, for instance, to the north-temperate wetlands of North America.
- The importance of stock watering ponds for satisfying the dietary requirements during breeding. Increased wetland drainage and water abstraction has increased the dependence of waterfowl on stock-watering ponds and dams in many areas of South Africa. However, the suitability of these waterbodies for satisfying the breeding requirements of waterfowl is poorly understood. Diurnal activity budgets indicated that White-faced Ducks were able to satisfy most of their daily dietary requirements during crepuscular foraging flights to neighbouring Panicum schinzii fields. While most species of north-temperate breeding ducks spend large portions of the day aquatic foraging, male and female White-faced Ducks foraged on stock-ponds for only 4.0% and 9.8% of the day, respectively. Low foraging effort is also indicative of a low aquatic resource availability on stock watering ponds, relative to north-temperate wetlands. While stock ponds provided scarce foraging opportunities, they did supply habitat for comfort-related activities, a critical requirement of waterfowl.
- Sexual differences and diurnal changes in activity patterns during breeding. Activity budgets were also performed to study the influence of perennial monogamy and high diurnal temperatures on activity patterns, and subsequently make comparisons with north temperate species which have seasonal pair bonds and breed where summer temperatures are less extreme. Male White-faced Ducks spent less time in comfort-related activities and more time alert than females. Increased alert behaviour by breeding males can be attributed to the fact that they invest considerable time in mate defence and vigilance, which serves to protect their reproductive investment, while enabling females to spend more time in comfort-related activities. There was an insignificant difference in foraging effort between male and female White-faced Ducks, which is in direct contrast to all foraging studies of pre-incubating North American anatids. Retention of pair bonds throughout the annual cycle may be advantageous to monogamous females, because paired females have been shown to forage more efficiently and for longer periods than unpaired females. However, while female White-faced Ducks may have been more efficient terrestrial foragers than males, we hypothesize that small sexual differences in foraging effort relative to north-temperate species is because, being primarily herbivorous, females do not spend a long time searching for macroinvertebrates. Thermal stress associated with high summer temperatures in Northern Province apparently influenced daily activity patterns because feeding, locomotion, courtship, and aggression were rarely observed during the hottest parts of the day.
- Timing and chronology of the prebasic moult in relation to Reproduction. While Dendrocygnids are the only tribe of ducks that have retained the ancestral pattern of one moult per cycle, this single annual moult has never been studied or described. Therefore, the intensity and chronology of feather replacement was quantified on collected birds in order to describe the prebasic moult in breeding birds and to determine if White-faced Ducks replace contour feathers during ovulation and incubation. Adult males and females were moulting in most feather areas when they arrived on the breeding grounds in December and January and continued to do so during rapid follicular growth. Both sexes moulted at very low levels during reproduction; males had a higher moult intensity score than females during both laying and incubation. Contour-feather replacement peaked during brood rearing and post-breeding and was followed by the loss of flight feathers. Most feather areas moulted intensively during the latter stages of wing-feather growth and continued to do so after birds had regained flight capabilities. While White-faced Ducks are capable of replacing contour feathers during brood rearing, the low protein content of their diet and high nutrient costs associated with reproduction apparently necessitate the separation of intense feather replacement and both laying and incubation. Shared incubation and brood rearing costs, perennial monogamy and lack of a breeding plumage could be the selection pressures leading to similar inter-sexual moult patterns and intensities. Their single annual moult and lack of a breeding plumage provides the temporal latitude for White-faced Ducks to prolong moult over a large portion of the annual cycle, while the low protein content of their diet possibly necessitates it.
- Relative importance of endogenous and exogenous lipid and protein during breeding. The endogenous nutrient (protein and lipid) reserve dynamics were quantified on birds collected throughout all stages of reproduction. The purpose of which was to ascertain if males and females transport endogenous reserves from wintering areas and to determine what proportions of reproductive nutrient requirements are satisfied through catabolization of stored nutrients and directly through dietary intake. Wintering areas are important sites for reproductive nutrient reserve acquisition because males and females arrived on the Nyl River floodplain breeding area with large lipid reserves and neither stored additional lipid after arrival. This enabled females to begin laying shortly after arrival. an important adaption given the ephemeral nature of most aquatic habitats in Northern Province, South Africa. Females catabolized 37g of their lipid deposit (LDEP)(wet skin+visceral fat+abdominal fat) and 26g their size adjusted protein deposit (AdjPDEP)(breast muscle+leg muscle+gizzard) during rapid follicular growth and ovulation, which accounted for 69.8, and 47.7% of the lipid and protein requirements of egg laying, respectively. Males catabolized 19g of LDEP and 18.4g of AdjPDEP between arrival and termination of laying. Females lost only 2% of their body mass during incubation and 0% during brood rearing. Males may have been less attentive than females during incubation and brood rearing as their body mass increased by 7% and 4% during these stages, respectively. Despite the fact that their breeding diets are high in fat and low in protein, females satisfied a large portion of the lipid requirements of clutch formation endogenously, suggesting that lipid may be limiting to White-faced Ducks during ovulation. Bi-parental care decreases the reliance of female White- faced Ducks on stored nutrient reserves during incubation, thereby enabling them to allocate large amounts of stored nutrients to egg laying.
- Effects of harness-attached 3Og satellite transmitters on affect activity patterns, body mass, skin, or feathers of captive White-faced Ducks. Given the contradictory reports of the effects of transmitters on waterfowl, the possibility that transmitters might adversely affect birds, and the extremely high costs of miniature satellite PTT'S, scientists have an ethical and practical obligation to ensure that PTTs and the attachment technique utilized are suitable for the species being studied. Consequently, experiments using captive birds were performed to determine the most suitable harness-attachment technique and to ensure that teflon harness-attached PTT's do not adversely affect the activity patterns or condition of White-faced Ducks. While transmitter affixed birds spent a slightly larger portion of the day preening (10,6%) than control birds (7,1 %), teflon harness-attached 3Og satellite PTTs had a minor effect on activity patterns and did not adversely influence the body mass, skin or feathers of captive White-faced Ducks. Therefore, teflon harness-attached satellite PTT's were deemed suitable for tracking the movement patterns of White-faced Ducks.
- Winter movement patterns and wetland use by White-faced Ducks from the Nyl River floodplain. In an attempt to identify movement patterns and wintering sites of the Nyl River floodplain population of White-faced Ducks, two post-breeding birds were captured on the Nyl River floodplain and fitted with 30 gram satellite PTT's using the teflon-harnessing technique that was perfected in captivity. Both birds relocated less than 85 km to heavily irrigated agricultural regions and spent the winter on irrigation ponds. The breeding population size of White-faced Ducks on the Nyl River floodplain during a given year is defined by rainfall and extent of flooding. Therefore, we suggest that during high flood years many birds make extensive movements to breed on the floodplain, whereas those that arrive and breed during dry years are primarily those that winter in close proximity. However, this hypothesis requires further study during successive years of variable rainfall. While on and adjacent to the floodplain breeding area, distance between successive weekly locations for the two birds was 17.0 and 21.0 km. After birds relocated to agricultural areas, distances dropped to 4.8 and 6.2 km, presumably due to an availability of agricultural grains. The relatively long weekly movements while on the Nyl River floodplain are indicative of a failing food supply on the floodplain and the availability of agricultural grains on the wintering area. Since many stock-watering ponds on and adjacent to the Nyl River floodplain contain water throughout the winter dry season, a time of limited native seed availability, the primary motivation for leaving the floodplain is apparently a lack of food availability. Therefore, we suggest that prior to European settlement the entire population of White-faced Ducks breeding in semi-arid areas would have been displaced to sub-tropical and riverine regions where winter food availability is presumably higher, even during relatively wet years. Therefore, provision of a predictable, highly nutritious, food source (agricultural grains) and fairly permanent waterbodies enables at least a portion of the Nyl River population of White-faced Ducks to winter in close proximity to breeding areas and to be more fixed and predictable in their annual movements (i.e. more migratory and less nomadic, dispersive, facultative).
- Summer Movement patterns and wetland use by White-faced Ducks in Northern KwaZulu-Natal In an attempt to identify movement patterns and breeding sites of the Ozambini Pan wintering population of White-faced Ducks, two birds were captured and fitted with 30 gram satellite PTT's using the teflon-harnessing technique that was perfected in captivity. Unfortunately both transmitters malfunctioned well before their one year life expectancy. Both birds remained within 30km of the Ensumu Pan release site and used seasonally flooded riverine areas and ephemeral and perennial pans. These localized movements suggest that the coastal plain of KwaZulu Natal may need to be managed for the full life cycle requirements of White-faced Ducks. However, results from this portion of the study are inconclusive and we suggest that the movements of White-faced Ducks that winter in this region require further study.
- Food consumed during winter/spring in northern KwaZulu Natal. Adult and juvenile White-faced Ducks were collected to determine what dietary items are most important during winter/spring in northern KwaZulu-Natal. White-faced Ducks were collected on a naturally occurring pan (Yengweni pan) and a large water storage dam (Pongolapoort Dam) during early winter (June), late winter (August) and spring (October). While the proportions of individual foods varied throughout the winter/spring and between the two collection sites there were no sex or age related dietary differences at either of the sites. The seeds of marginal (>99% aggregate dry mass) and aquatic plants (>97%) were the principal dietary items of birds using the Pongolapoort Dam and Yengweni pan, respectively. While extensive submerged plant communities permit birds to consume large quantities of aquatic seeds on the Yengweni Pan, large fluctuations in water level on the Pongolapoort Dam impede aquatic plant establishment, forcing birds to forage elsewhere. Invertebrates were minor dietary items (<1.0%) and although birds tended to consume higher levels of animal matter during the wing-moulting period, invertebrate consumption was not correlated with moult intensity. Results suggest that White-faced Ducks wintering in northern KwaZulu-Natal are much more dependant on aquatic food sources than birds breeding on the Nyl River floodplain.
- Activity patterns of White-faced Ducks throughout the winter/spring in northern KwaZulu-Natal The nocturnal and diurnal activity patterns of non-breeding White-faced Ducks were quantified on semi-permanent pans in northern KwaZulu-Natal during the winter and spring. The primary purpose of this portion of the study was to ascertain the importance of sub-tropical pans to foraging White-faced Ducks and determine if birds increase their foraging intensity during peak flight feather replacement. Increased foraging intensity during early winter (44%) and spring (50%) corresponded with peak wing-feather replacement. Fifty-three percent of foraging activities occurred nocturnally. Comfort related activities (sleep, loaf, preen) represented 35, 57, and 29% of early winter, late winter and spring activities, respectively. Time spent in locomotion (walk, swim, fly) decreased as winter progressed and was lowest during spring, whereas alert behaviours and aggression increased. White-faced Duck activities were more strongly correlated with flock size and ambient temperature than several other measured climatic variables. The activity patterns of wintering White-faced ducks are apparently influenced by current feather replacement costs and the energetic and nutrient costs of past and future reproductive output.
- Moult patterns during winter/spring. The intensity and chronology of feather replacement was quantified on birds collected in northern KwaZulu-Natal in order to determine if White-faced Ducks extend the prebasic moult throughout the winter, and to determine if they replace flight feathers on wintering areas. Adult and immature birds moulted contour feathers throughout the entire winter and spring and 10% of early winter, 3% of late winter and 34% of spring collected adults were in the later stages of flight feather replacement. While the replacement of flight feathers on breeding areas following reproduction is ubiquitous among north-temperate breeding ducks, the ephemeral nature of breeding habitats in many areas of South Africa apparently forces most White-faced Ducks to replace flight feathers on wintering areas. While the fact that White-faced Ducks are seasonally and sexually monochromatic permits the temporal extension of the prebasic moult, the low protein content of the diet may necessitate it.
- Endogenous reserve dynamics during winter/spring. The endogenous nutrient (protein, lipid and calcium) reserve dynamics were quantified on White-faced Ducks collected in northern KwaZulu-Natal to determine if they catabolize endogenous reserves during flight feather replacement, and if and when adults and immatures acquire endogenous reserves on wintering areas. Late wing-moulting males and females had 28.9g (51%) and 18.6g (34%) less lipid than non wing-moulting males and females, respectively, however; they did not exhibit differences in size adjusted protein (ADJprotein) or ash (ADJash) during this time. Adult males, adult females and immatures had similar body, ADJprotein, lipid, and ADJash mass's during early winter. Adult ADJprotein and lipid increased between early and late winter, followed by declines in lipid during spring, which can be attributed to heightened flight-feather replacement at this time. Immatures exhibited marked declines in all measured carcass components between early winter and spring. As White-faced Ducks are most aggressive during spring, we attribute this decline to dominant individuals (adult males and possibly females) limiting immature (subordinate) access to preferred feeding sites and displacement during pair-bond formation and strengthening. If White-faced Ducks acquire lipid on wintering wetlands, they do it immediately before departure for breeding areas. Due to the low probability of extreme cold spells, it is presumably not adaptive for White-faced Ducks to store and maintain large fat stores throughout the winter. However, we suggest that flight feather replacement and competition for food may preclude White-faced Ducks from acquiring large fat reserves anyway.
- A comparison of activity patterns, foraging strategies, nutrient reserve dynamics, and moult chronologies of birds wintering in Northern Province and northern KwaZulu-Natal Many White-faced Ducks have responded to European-colonization by over-wintering on irrigation ponds in Northern Province. The body condition, diet, activity patterns and moult intensity of birds was studied prior to departure for breeding areas and compared to birds that over-winter in northern KwaZulu-Natal to determine the selective advantages/disadvantages of wintering in either of the two areas. There were no sexual or age-related differences in the proportion of individual food items consumed, 96.6% of which was corn. By foraging on neighbouring agricultural fields, birds were able to spend large portions (80%) of the day/night in comfort related activities (preen, loaf, sleep) and little time foraging (8.8%) while on ponds. In contrast, birds that winter in northern KwaZulu-Natal consume native seeds and spend large portions (approximately 50%) of the day and night aquatic foraging. With the exception of flight feathers, birds were moulting moderately to heavily in all twenty feather areas and at similar intensifies to birds during spring in northern KwaZulu-Natal. However, a smaller proportion of the birds collected in Northern Province were replacing flight-feathers (3%) than birds collected during spring in northern KwaZulu-Natal (34%). White-faced Ducks in Northern Province had higher levels of carcass protein, lipid, and ash than birds collected during late winter and spring in northern KwaZulu-Natal, the disparity of which was particularly pronounced for immatures. Therefore, the consumption of corn and associated low foraging effort conveys nutritional benefits to birds that over-winter in agricultural regions near breeding areas. Being in better condition during spring and wintering in close proximity to breeding areas presumably reduces immature mortality rates while enabling adults to arrive on breeding areas and initiate egg laying earlier. Therefore, we suggest that the population increase and range expansion of White-faced Ducks in South Africa is at least partially due to intensive agriculture.
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